Charge/discharge protection circuit

ABSTRACT

The invention refers to a charge/discharge protection circuit for a rechargeable battery, where the protection circuit is integrated on a single chip, including the fusible link, the load current switch and the short-circuit switch. This is achieved by dividing the functions of the fusible link, the load current switch, and the short-circuit switch into in parallel arranged T-sections, each of which is designed for only a fraction of the nominal load so that each of the easily integrated fuse segments carry only the respective fraction of the nominal current. It is important that the entire protection circuit or its control logic will not be destroyed before through an unduly high over-voltage, in which case the sequential melting of the fuse segments would no longer be guaranteed. This is handled by a semiconductor switch which short-circuits the over-voltage immediately.

[0001] The invention refers to a charge/discharge protection circuit for a rechargeable battery comprising at least one rechargeable cell, with a control logic which opens or closes a load current switch depending on the magnitude of the voltage at the battery terminals, the voltage at the charge/discharge terminals of the protection circuit, and the charge or discharge current. Where that control logic comprises an over-voltage detector which produces a control signal when reaching a voltage limit which depends on the electric strength as determined by the protection circuit. This control signal closes a short-circuit switch which connects the battery terminals via a load current switch in series with a fusible link.

[0002] Such a circuit is deemed known from the this-side DE-A-101 03 3362, which is expressly referenced herewith. In particular, this circuit can be disposed between a single lithium-ion cell and the electronics of a mobile phone. It can be produced at favorable cost with a significantly lower electric strength than inherently required in a worst case, because the short-circuit switch closes when reaching an appropriately determined voltage limit. The resulting short circuit current thus safely destroys the fusible link so that the cell is protected from a dangerous overcharge. In contrast to all other circuit elements however, the fusible link cannot be integrated, or only at high cost, on an IC because it has to respond very reliably at a tightly toleranced nominal current.

[0003] The task of this invention is based on the requirement to create a protection circuit of the above described type which can be integrated on a single chip including the fusible link. This task is inventively solved by designing a series circuit comprising the load current switch and the fusible link for a fraction 1/n of the nominal current and shunting n of such series circuits in parallel, that the short-circuit switch consists of n switch segments, each of which coupled between the node of the load current switch and the fusible link of the series circuit x (x=1, 2 . . . n) and the opposite battery connection, that the over-voltage detector creates in sequential succession for each switch segment x a separate control signal, and that there is disposed a semiconductor switch in parallel to the charge/discharge terminals of the protection circuit, which the over-voltage detector switches conductively when reaching the predetermined voltage limit.

[0004] The central idea of the invention is, therefore, to create the condition where the fuse segments in the case of an over-voltage are reliably destroyed and where the destruction proceeds sequentially from the first to the last fuse segment. This is achieved by dividing the functions of the load current switch, the fusible link, and the short-circuit switch into in parallel arranged T-sections, each of which is designed for only a fraction of the nominal load so that each of the easily integrated fuse segments carry only the respective fraction of the nominal current. It is important in this case that the entire protection circuit or its control logic will not be destroyed before through that unduly high over-voltage, in which case the sequential blowing of the fuse segments would no longer be guaranteed. This is taken care of by the semiconductor switch which immediately short-circuits a recognized over-voltage.

[0005] Appropriately, the over-voltage detector receives as input voltage the voltage at the charge/discharge terminals (claim 2). In principle, the over-voltage detector could also receive as input voltage the voltage via the parallel circuit of the opened load current switches, the load current switches having opened due to the over-current caused by the previous over-voltage.

[0006] Appropriately, at least those circuit components of the control logic generating the control signals for the switch segments receive their supply voltage from an auxiliary voltage source, in particular from a charged buffer capacitor (claim 3). This safeguards the function of the protection circuit when the outer over-voltage is short-circuited by the semiconductor switch or if it is so high that it has lead to a destruction of other intended parts of the control logic.

[0007] The over-voltage detector may in particular comprise a bistable flip-flop circuit, which switches to its second stable state when the predetermined voltage limit is reached (claim 4), so that the sequential closing of the switch segments of the short-circuit switch is initiated when the predetermined voltage limit is reached during even briefly.

[0008] The over-voltage detector may comprise a clock generator followed by a shift register of at least n levels, which, when reaching the predetermined voltage limit, delivers sequentially the control signals for the switch segments of the short-circuit switch (claim 5).

[0009] Alternatively to this method of producing the control signals, it is possible that each switch segment of the short-circuit switch has associated with it a voltage detector, which delivers a signal at its output when the voltage at this switch segment disappears as a result of the blowing of the associated fuse segment whereby the outputs of the voltage detectors are coupled to a logic circuit which comprises a clock generator and which, when exceeding the voltage limit, produces the control signal for the next in line switch segment only when the voltage detector of the preceding switch segment delivers an output signal (claim 6). The logic circuit can be realized, in particular, with an appropriate number of sequentially connected and with each other coupled flip-flops, as is well understood by those skilled in the art, or an appropriate number of short shift registers. Utilizing shift registers has the advantage in this instance that the control signals do not follow each other immediately, but have a short delay determined by the frequency of the clock generator and the number of shift register stages, thus allowing the current path created by the prior blowing of the fuse segment to cool down.

[0010] In particular, the semiconductor switch and/or the switch segments of the short-circuit switch may comprise two in series connected, parallel controlled single switches (claim 7), in order that these components may also be realized in a low-cost technology offering only low electric strength.

[0011] Preferably, all circuit components, with the exception of capacitors, are integrated on a chip (claim 8).

[0012] Embodiments of a protection circuit according to the present invention are shown schematically simplified in the drawings. Illustrated is:

[0013]FIG. 1 a block diagram of a first embodiment

[0014]FIG. 2 a simplified circuit diagram of the over-voltage detector in FIG. 1,

[0015]FIG. 3 a simplified circuit diagram of the clock generator and shift register in FIG. 1,

[0016]FIG. 4 a block diagram of a second embodiment, and

[0017]FIG. 5 a simplified circuit diagram of the logic circuit in FIG. 4.

[0018] The protection circuit illustrated in FIG. 1 is located as an integrated circuit between the terminals of a Li-Ion cell 1 and two charge/discharge terminals 50 and 60, paralleled to which is a filter-capacitor 40 as protection against steeply sloped voltage increases. The protection circuit has the outer connections Corn, +Batt and Out as well as +V_(H). The latter is connected via a buffer capacitor 2 with the reference potential Corn. Buffer capacitor 2 is normally charged to the battery voltage via semiconductor switch 4 (indicated as a diode). If this supply voltage drops away then semiconductor switch 4 opens and the buffer capacitor 2 supplies, via line 5, power for a time for essential parts of the integrated circuit. A charge pump circuit may also be used in place of semiconductor switch 4, where the charge pump circuit charges the buffer capacitor 2 to the intended maximum gate voltage as dictated by the technology of the integrated circuit.

[0019] A current sensing resistor R_(sense) is placed between Out and +Batt in series with a circuit arrangement, which comprises a plurality of parallel coupled series circuits, in turn each comprising a load current switch 10 ₁, 10 ₂, . . . 10 _(n) and an integrated fusible link 11 ₁, 11 ₂, . . . 11 _(n). Each connection node between a load current switch 10 _(x) and a fusible link 11 _(x) is connected to the reference potential Corn via its own short-circuit switch 12 ₁, 12 ₂, . . . 12 _(n).

[0020] A control logic, pictured in simplified form within dash-dotted block 6, generates by generally known means through differential amplifiers D1 and D2 a battery undervoltage signal UV and a battery over-voltage signal OV, provided the voltage of the Li-Ion cell is outside of the predetermined tolerance range. The control logic further generates from the voltage drop across R_(sense) by means of differential amplifiers D3, D4 the output signals OCD, OCC, respectively, when reaching the maximum charge current or the maximum discharge current, respectively. Output signals UV, OV, OCC and OCD of differential amplifiers D1 to D4 are coupled into OR-gate OD which supplies an output signal F1 which opens in each of the mentioned cases all load current switches 10 ₁ through 10 _(n).

[0021] The voltage V_(out) is applied at the same time to the input of an over-voltage detector OVD which generates an error signal F2 in case of an over-voltage. Error signal F2 also closes at the same time two semiconductor switches 7, 8 which are connected in series between Com and Out, generates via OR-gate OD the error signal Fl to open the load current switches 10 ₁ through 10 _(n), and activates a clock generator CLK. Coupled to the clock generator CLK is a shift register SR which then sequentially delivers error signals F3 [1:n] for successive closing of the short-circuit switches 12 ₁ through 12 _(n). This has the consequence that, one after the other, fuses 11 ₁ through 11 _(n) blow, so that Li-Ion cell 1 will be cut off from Out after a few milliseconds to a few 100 milliseconds. At least until the last fuse 11 _(n) has blown, control logic 6 will be fed by the buffer capacitor 2, if necessary.

[0022]FIG. 2 illustrates an embodiment of the over-voltage detector. The designations of the outer connections correspond with those of FIG. 1. Placed between Out and Corn is a series circuit comprising first trigger element 21 and a resistor 22 as well as a second trigger element 23 and a second resistor 24. Trigger elements 21, 23 are only illustrated symbolically as the series connection of a zener diode and a back current diode wired back-to-back. When exceeding the predetermined voltage limit in the positive or negative direction the corresponding trigger element becomes conductive and with it the respective, following MOS transistor T1 or T2, which in turn sets to “0” or “L” the following Schmitt-Trigger ST1, ST2, respectively. The outputs of the Schmitt-Triggers are coupled via Inverter I with the inputs of a NAND-gate 25, whose output in turn is connected with the clock input of a D flip-flop 26. Its D-input is normally at “1” or “H”. The output Q of flip-flop 26, therefore, goes to “H” when at the clock input an impulse arrives from OR-gate 25. The over-voltage detector behaves, therefore, like a bistable flip-flop. The output signal “H” represents the signal F2.

[0023]FIG. 3 illustrates clock generator CLK with the shift register SR coupled to it as shown in FIG. 1. Clock generator CLK is started when signal F2 is applied to its start input EN and supplies at its output clock pulses clk, which are applied to the clock input of shift register SR. At the same time the latter receives at its signal input XR the signal F2 and at its D-input the signal “1” or “H”. Shift register SR outputs Q₁, Q₂, . . . Q_(n), equal to signal F3 [1:n], thus switch sequentially from “L” to “H” in accordance with the clock signal, thereby delivering sequentially following control signals for closing of the switch segments 12 ₁ through 12 _(n) in FIG. 1.

[0024]FIG. 4 illustrates a block diagram of a second embodiment of the protection circuit.

[0025] Having a similar arrangement as the embodiment of FIG. 1, it differs from that because voltages V₁ through V_(n−1) picked off at the short-circuit switches 12 ₁ through 12 _(n) are selected and then supplied to a logic circuit LS, replacing the shift register SR of FIG. 1.

[0026] This logic circuit LS supplies sequentially the control signals F3 [1:n] and is illustrated in FIG. 5. It receives those in FIG. 4 named voltages or signals and includes for each switch segment 12 ₁ through 12 _(n), respectively the voltages V₁ through V_(n−1) picked off from each, a voltage detector ST illustrated as a Schmitt-Trigger, and a number of D-flip-flops DFF₁ through DFF_(N) whose number equals the number of switch elements. The clock generator CLK, preferably operating at a higher frequency than the one of FIG. 1, will be started by signal F2 as before and delivers clock signals clk to the clock input of DFF₁. Because error signal F2 is applied at the same time to all inputs XR of the D-flip-flops, thereby enabling them, the first clock pulse CLK forces output Q of DFF₁ to “1” and thereby generates the first control signal F3 [1] which closes switch segment 12 ₁. As long as its own fuse segment 11 ₁ is still intact, the voltage V₁ is then >0 V (typically 0.5 to 1.5 V). The corresponding voltage detector therefore supplies an output signal “1”. When fuse 11 ₁ has blown a short time after the closing of the switch segment then the voltage switches to V₁<0.5 V. The output signal of the voltage detector goes to “0”. The following inverter I generates out of it a “1” for the second input of AND-gate U₁. Applied to the first input is signal F3 [1], so that the output of the AND-gate U₁ also switches to “1”. This output is coupled to the D-input of the next flip-flop DFF₂, which, in corresponding manner, supplies with the next rise of the clock signal clk the control signal F3 [2] etc.

[0027] A further improvement (but not illustrated in the drawings) consists of replacing each of the flip-flops DFF by a short shift register, e.g. with three successive, appropriately coupled D-flip-flops. Thus, each control signal will be generated after a time delay of e.g. three clock cycles, after the voltage Vx at the respective switch element has dropped, due to the blowing of the respective fuse element, below the switching threshold of the voltage detector. During this time delay the current path of the just-before blown fuse element can cool down. This method avoids having a retroactive effect on the response characteristics of the physically closely spaced adjacent, and still intact, fuse element.

[0028] Should, in the meantime, the current path of this fuse element become again conductive when the before melted-away metal, typically aluminum, solidifies, then this would be recognized by its own voltage detector in the same manner as before and a new blowing of this fuse element would be initiated. This further increases the reliability of the protection function of the total circuit. 

1. A charge/discharge protection circuit for a battery comprising at least one rechargeable cell (1), with a control logic (6), which opens or closes a load current switch depending on the magnitude of the voltage at the battery terminals (+Batt), the voltage at the charge/discharge terminals (50, 60) of the protection circuit, and the charge or discharge current, which control logic (6) comprises an over-voltage detector (OVD), which generates a control signal (F3) when reaching a voltage limit which depends on the electric strength as determined by the protection circuit, which control signal closes a short-circuit switch (12), which connects the battery terminals via a fusible link (11) in series with a load current switch, thus characterized, that a series circuit comprising the load current switch (10 [1:n]) and the fusible link (11 [1:n]) is designed for a fraction 1/n of the nominal current and where n of such series circuits are shunted in parallel, that the short-circuit switch comprises n switch segments (12 [1:n]), each of which coupled between the node of the load current switch and the fusible link of the series circuit x (x=1, 2 . . . n) and the opposite battery connection (Com), that the over-voltage detector (OVD) creates for each switch segment (12 [1:n]) in sequential succession a separate control signal (F3 [1:n]), and that there is disposed a semiconductor switch (7, 8) in parallel to the charge/discharge terminals (50, 60) of the protection circuit, which the over-voltage detector (OVD) switches conductively when reaching the predetermined voltage limit.
 2. Protection circuit according to claim 1, thus characterized that the over-voltage detector (OVD) receives as input voltage the voltage at the charge/discharge terminals (50, 60).
 3. Protection circuit according to claim 1 or 2, thus characterized that at least those circuit components of the control logic (6) which generate the control signals (F3) for the switch segments (12) receive their supply voltage (+V_(H)) from an auxiliary voltage source, in particular from a charged buffer capacitor (2).
 4. Protection circuit according to claim 3, thus characterized that the auxiliary voltage source comprises a buffer capacitor (2), which is charged via a semiconductor switch (4) to the battery voltage, where the semiconductor switch (4) opens when the battery voltage falls below a predetermined value or when the over-voltage detector (OVD) produces a control signal upon exceeding the voltage limit.
 5. Protection circuit according to claim 3, thus characterized that the auxiliary voltage source is a buffer capacitor (2), which is charged via a charge pump to the maximum allowable gate voltage for the control logic (6).
 6. Protection circuit according to one of the claims 1 to 5, thus characterized that the over-voltage detector (OVD) comprises a bistable flip-flop circuit (26) which switches to its second stable state when the predetermined voltage limit is reached.
 7. Protection circuit according to one of the claims 1 to 6, thus characterized that the over-voltage detector is followed by a clock generator (CLK) and an at least n level shift register (SR), whose levels supply, when reaching the predetermined voltage limit, sequentially the control signals (F3) for the switch segments (12).
 8. Protection circuit according to one of the claims 1 to 7, thus characterized that each switch segment (12) of the short-circuit switch has associated with it a voltage detector (ST), which delivers a signal at its output when the voltage at this switch segment disappears as a result of the blowing of the associated fuse segment (11) and that the outputs of the voltage detectors are coupled to a logic circuit which comprises a clock generator (CLK) and which, when exceeding the voltage limit, produces the control signal (F3) for the next in line switch segment only when the voltage detector of the preceding switch segment delivers an output signal.
 9. Protection circuit according to one of the claims 1 to 8, thus characterized that the semiconductor switch and/or the switch segments comprise two in series connected, parallel controlled single switches.
 10. Protection circuit according to one of the claims 1 to 9, thus characterized that, with the exception of capacitors, all circuit components are integrated on a chip. 